Step 1

In a large skillet, fry diced bacon over medium heat until crispy, about 8-10 minutes. Remove from skillet and place on a paper towel-lined plate to drain. Remove most of the fat from the skillet, leaving some to cook the onions in. Cook onions over medium heat until translucent, about 7-10 minutes

Step 2

Preheat oven to 350° F. Grease a 13x9 pan and place aside.

Step 3

In a large bowl, whisk together the eggs, milk, salt, pepper, and mustard powder until fully combined.

Step 4

Cut or break apart croissants into bite-sized pieces. Spread these in the bottom of the 13x9 pan. Sprinkle ¼ cup each of cheddar and gruyere cheese over top of croissants, and then sprinkle bacon bits and onions over top. Pour egg mixture on top. Gently mix this up so that the croissants are completely covered with the egg mixture. Finally, sprinkle the last ¼ cup of each cheese on top.

Step 5

Bake the casserole for 30-40 minutes, or until the eggs in the center are fully cooked. If the casserole starts to brown too much, place foil on top around the 30-minute mark.

Step 6

Remove from the oven and allow to cool for 5-10 minutes. Serve warm.